Off-highway implements, such as large heavy-duty self-propelled tractors and the like, typically incorporate both multi-speed and multi-range transmissions which are arranged in combination with each other to effectively act as a single transmission assembly which provides a wide range of operating conditions for the implement. Independent of the operating speed range chosen by the operator, conventional transmission assemblies for off-highway implements are selectively operable in forward, reverse and neutral modes of operation.
Until recently, operation of the implement in either a forward or reverse operating mode was effected by the operator using a fulcrumed foot pedal having linkage mechanisms into the transmission assembly. Later, a hand lever and electrical control arrangement, as disclosed in Case Corporation U.S. Pat. No. 4,966,044, was developed to facilitate controlling the forward/reverse mode of operation for the implement. This patented arrangement helped the operator control the implement as there are many different functions an operator must perform substantially simultaneously while operating heavy duty implements or equipment which makes it difficult to use the heretofore known foot pedal actuated devices for controlling the direction of the implement.
The hand lever arrangement disclosed in U.S. Pat. No. 4,966,844 uses a two-position over-center arrangement for controlling the direction of the implement. One position of the hand lever selects a forward mode of implement operation and the other hand lever position selects a reverse mode of operation for the implement. The hand lever was resiliently biased such that the lever would automatically move to condition the implement for either a forward or reverse mode of operation depending upon the over-center position of the lever.
To move the hand lever from one position to the other position and back, the hand lever had to be purposefully moved in two different directions. That is, to move the hand lever from one position to another required the operator to purposefully move the hand lever both upwards and sideways. In this manner, it was assured that the hand lever would not inadvertently move from one position to another position thus inadvertently changing the directional mode of operation of the implement. The hand lever is also connected to a electrical switch that detects the position of the lever and engages the transmission assembly to operate the implement in the mode selected by the operator.
The multi-speed transmission of the transmission assembly typically includes a shift mechanism which selectively allows the speed transmission to operate in any of four different speeds. For each speed, the multi-range transmission of the transmission assembly is selectively operable under the influence of another shift mechanism in any of four different ranges. Thus, the transmission assembly of the off-highway implement is selectively operable in any of sixteen different speed ranges. As will be appreciated by those skilled in the art, the gearing in the multi-speed and multi-range transmissions is such that the fourth speed range of the transmission assembly develops the highest ground speed for the implement. Because of the relatively high ground speeds obtainable by the implement when the transmission assembly is conditioned in a fourth speed range, it is inadvisable to allow the implement to operate in reverse when the transmission assembly is conditioned in a fourth speed range.
A cab region of the off-highway implement contains an operator's seat adjacent to which are the various shift levers for conditioning the transmission assembly, the hand mechanisms for setting the appropriate mode of operation for the implement, a steering column, and other implement control devices. As will be appreciated by those skilled in the art, the cab region of a typical off-highway implement has relatively tight space constraints. Upon leaving the cab region, an operator typically places the transmission shift mechanism in a neutral position corresponding to a neutral condition for the transmission assembly. The hand lever of the heretofore known directional control system, however, is normally biased into either a forward or reverse position. Because of the confined space constraints in the cab region of the implement, it is not unusual for an operator to inadvertently move either transmission shift mechanism from its neutral position upon entering or leaving the cab region. The problems with inadvertent displacement of either shift mechanism from a neutral position whereby conditioning the transmission assembly in other than a neutral condition while the directional control assembly is positioned to condition the implement for forward or reverse modes of operation are apparent and well known in the art especially when the engine of the implement is being started.
Thus, there is both a need and a desire for a directional control system for conditioning an off-highway implement in either forward, reverse or neutral modes of operation and which prevents the implement from starting when a directional selector of the directional control system is in other than a neutral position.